The present invention relates generally to integrated circuit design and fabrication. Specifically, the present invention relates to semiconductor fuses, methods for fabricating the semiconductor fuses, methods for using the semiconductor fuses, and semiconductor devices containing the semiconductor fuses.
Computers typically have various types of devices which store data, such as memory devices. One type of memory device is a read only memory (ROM) device in which data is permanently stored and cannot be overwritten or otherwise altered. Thus, ROM devices are useful whenever unalterable data or instructions are required. ROM devices are also non-volatile devices, meaning that the data is not destroyed when power is shut off. ROM devices are typically programmed during fabrication by making permanent electrical connections in selected portions of the memory device. One disadvantage of ROM devices is that their programming is permanently determined during fabrication and, therefore, can only be changed by redesign.
Another type of memory device is a programmable read only memory (PROM) device. Unlike ROM devices, PROM devices are programmable after their design and fabrication. To render them programmable, PROM devices are typically provided with an electrical connection in the form of a fusible link (fuse). There are a considerable number of fuse designs used in PROM devices, such as those disclosed in IEEE Transactions on Electron Devices, Vol. 33, No. 2, p. 250-253 (February 1986), and in U.S. Pat. Nos. 5,589,706, 4,491,860, 5,625,218, 4,796,075, and 4,740,485, the disclosures of each of which are incorporated herein by reference. Perhaps the most common fuse design is a metal or polysilicon layer which is narrowed or “necked down” in one region. To blow the fuse, a relatively high electrical current is driven though the metal or polysilicon layer. The current heats the metal or polysilicon above its melting point, thereby breaking the conductive link and making the metal layer or polysilicon discontinuous. Usually, the conductive link breaks in the narrowed region because the current density (and temperature) is highest in that region. The PROM device is thus programmed to conducting and non-conducting patterns, thereby forming the 1 or 0 comprising the data stored in the memory device.
Rather than employing an electrical current, a laser can be employed to blow the fuses. Using lasers instead of electrical current to blow fuses, however, has become more difficult as the size of memory devices decreases. As memory devices decrease in size and the degree of integration increases, the critical dimensions (e.g., fuse pitch) of memory cells become smaller. The availability of lasers suitable to blow the fuse becomes limited since the diameter of the laser beam should not be smaller than the fuse pitch. Thus, the fuse pitch, and the size of memory devices, becomes dictated by the minimum diameter of laser beams obtainable by current laser technology.
The ability of electrical currents to blow fuses could aid in adapting fuses for a variety of applications, such as redundancy technology. Redundancy technology improves the fabrication yield of high-density memory devices, such as SRAM and DRAM devices, by replacing failed memory cells with spare ones using redundant circuitry, which is activated by blowing fuses. Using laser beams to blow the fuses limits the size and, therefore, the number of memory devices as explained above since the diameter of a conventional laser beam is about 5 microns. Using electrical currents instead to blow fuses, therefore, has a greater potential for high-degree integration and decreased size of memory devices.
The present invention generally provides fuses for integrated circuits and semiconductor devices, methods for making the same, methods of using the same, and semiconductor devices containing the same. The semiconductor fuse of the present invention contains two conductive layers, an overlying layer and an underlying layer, disposed on an insulating substrate. The underlying layer comprises a refractory metal nitride, such as titanium nitride, and the overlying layer comprises tungsten silicide. The semiconductor fuse may be fabricated during manufacture of local interconnect structures containing the same materials.
The present invention includes a semiconductor fuse comprising an insulating substrate, a titanium nitride layer disposed over the insulating substrate, and a tungsten silicide layer disposed over the titanium nitride layer. The insulating substrate maybe an isolation region disposed on a silicon or other semiconductor substrate. The titanium nitride layer and the tungsten silicide layer may have a similar pattern, which includes a neck portion located between terminal portions. The neck portion may be smaller in width than the terminal portions and may have a width of about 0.35 microns and length of about 3.5 microns. The inventive semiconductor fuse may be contained in an integrated circuit, either alone or with a local interconnect structure.
The present invention also includes a method of making a semiconductor fuse by providing an insulating substrate, forming a titanium nitride layer over the insulating substrate, and forming a tungsten silicide layer over the titanium nitride layer. The insulating substrate may be a field oxide region formed by thermally oxidizing a portion of a silicon substrate. The titanium nitride layer may be formed by depositing a layer of titanium and annealing the titanium in an atmosphere containing nitrogen or by depositing a layer of titanium in an atmosphere containing nitrogen. The tungsten silicide layer may be formed by chemical vapor deposition. The tungsten silicide layer may be patterned by a photolithographic pattern and etch process and the titanium nitride layer may then be patterned by a wet etch process using the patterned tungsten silicide layer as a hard mask. The method for making the semiconductor fuse can be incorporated into a method for making an integrated circuit containing the fuse alone or containing the fuse and a local interconnect structure.
The present invention also includes a method of using a semiconductor fuse by first providing a semiconductor fuse comprising a tungsten silicide layer and a titanium nitride layer disposed on an insulating substrate and having a neck portion located between terminal portions and then flowing sufficient electrical current to blow the fuse by causing the neck portion of the tungsten silicide layer to melt. When the neck portion has a width of about 0.35 microns and a length of about 3.5 microns, the electrical current sufficient to blow the fuse is about 5.5 mA and the leakage current of the blown fuse is about 1 nA.
By fabricating the semiconductor fuse with tungsten silicide and titanium nitride over an insulating substrate, the fuse of the present invention can be manufactured while also fabricating a local interconnect structure with the same materials. The inventive semiconductor fuse, which is used to program redundant circuitry, can be blown by electrical current rather than laser beams, thus allowing the fuse pitch to be smaller than the pitch of prior art fuses which are blown by laser beams. Another advantage of the inventive semiconductor fuse is that the fuse may be blown by less electrical current, on the order of less than half of the current required to blow conventional polysilicon fuses.